Adsorbents are commonly used to remove impurities or other constituents from liquids and gases. For example, activated charcoal finds wide usage in water purification and also in air and other gas purification, the function of the activated charcoal being that of an adsorbent for the impurities or other constituents desired to be separated of the fluid. As another example, granular activated silica gel is widely used as a desiccant, i.e. as a water moisture-adsorbent, as is also granular activated alumina.
For most applications of such adsorbents, it is desirable for economic reasons to thermally or otherwise treat the adsorbent, after it has become saturated, in order to remove from it the adsorbent materials, thereby to regenerate the adsorbent so that it can be re-used. Also, it is conventional to use the adsorbent in loose granuler form so as to provide maximum surface area. The loose granular adsorbent material is constrained to form a bed within a suitable container which might, for example, be a columnar vessel or, as another example, a cloth, metal screen or other porous bag or the like. The latter type container -- a porous bag or the like -- is generally used for static applications whereas the columnar vessel type container for the granular bed is generally used for applications where the fluid is flowed through the bed. In the case of the latter, the flow can be upwardly through the column, referred to as using the granular bed in an upstream mode, or alternatively, the flow can be downwardly through the column which is referred to as use in a downstream mode.
But whatever the precise form or mode of use, adsorbents in the form of loose granules have two serious disadvantages. The first disadvantage is that the loose granules, because they are movable with respect to each other, come into repeated forceful contact due to fluid flow currents and turbulence, vibration and other causes, thereby resulting in a fracturing and erosion of the granules to produce fines of the adsorbent material. As the proportion of fines increases at various points in the bed of granules, they clog the passages between the granules thereby causing increased turbulence and increased local points of pressure, and eventually the formation of channels through the bed of granules. This substantially reduces the effective amount of surface area to which the fluid being treated is exposed. Further, particularly where the bed is used in the upstream mode, the fines in the upper layers of the bed can become fluidized at points where the fluid being treated has acquired a higher than normal velocity, as at the head of a channel. As more and more fines are formed, many of them find their way out the top of the bed and are carried in the fluid stream, causing loss of adsorbent from the bed and contamination of the fluid, from which they must be eliminated by adding steps to the process, such as cycloning or screening, which costs money and reduces the overall efficiency of the process sequence. For this reason the upstream mode though otherwise generally more effective, is seldom used in place of the downstream mode.
Where the adsorbent granules are contained in a porous cloth or the like bag or other container, the fines gradually clog the passages near and through the bag or other container, thereby interfering with the free flow of the fluid.
The second serious drawback of using the adsorbent in loose granule form is that it makes difficult, and relatively expensive, the regeneration of the adsorbent. That is, the particulate adsorbent, in the form of the granules and the fines, must be removed as a loose mass from the column or other container and then not only treated to cause the regeneration or re-activation of the adsorbent material, but also sieved or otherwise fractionated to remove the fines. This not only necessitates an additional operation, but it also entails considerable loss of the adsorbent material in the form of the fines, which cannot be re-used unless reprocessed into granules, which involves even greater expense. A third and attendant drawback to the use of adsorbents in loose granule form is that the regeneration of the adsorbent material is a messy operation at best, and particularly where charcoal is the adsorbent, also a dirty operation.